Correctly substitute the internal @BUILD_ROOT@ token with MSVC backend

To me, this looks like a mistake in 976c9abc, but perhaps there's something
I don't understand going on here.

mesonbuild/backend/backends.py

@@ -837,7 +837,7 @@ class Backend:
         outdir = self.get_target_dir(target)
         if absolute_outputs:
             source_root = self.environment.get_source_dir()
-            build_root = self.environment.get_source_dir()
+            build_root = self.environment.get_build_dir()
             outdir = os.path.join(self.environment.get_build_dir(), outdir)
         outputs = []
         for i in target.get_outputs():